1. Field of the Invention
This invention relates to a trench gate lateral MOSFET structure. In particular, the present invention relates to a trench gate lateral MOSFET structure that has a high breakdown voltage and specific on-resistance lower than previous structures.
2. Description of Related Art
Conventional lateral power MOSFETs have a blocking voltage supported by the extension of a depletion region from a P-base/N-drift region junction toward a drain region. By optimizing the charge in the drift region, such devices can establish a uniform electric field along the surface of the device. However, because the average value of the field in such devices is approximately 1.2.times.10.sup.5 volts per centimeter, the separation between the P-base/N-drift region junction and the drain, to support a blocking voltage of 120 volts, must be approximately 10 microns. Further, because the voltage is supported laterally in the device, the cell pitch of the device is relatively large, which results in a relatively high specific on-resistance. For a breakdown voltage of 120 volts, the typical specific on-resistance of a conventional lateral power MOSFET is about 10 milliOhm-cm.sup.2. The best fabricated discreet vertical channel power MOSFETs have a specific on-resistance of about 3 milliOhm-cm.sup.2. Goals in the development of such power MOSFETs include reducing specific on-resistance for a given breakdown voltage while reducing the cell pitch of the device.
U.S. Pat. No. 5,142,640 to Iwamatsu discloses a lateral trench gate MOSFET. In the lateral trench gate MOSFET disclosed by Iwamatsu, the trench usually extends within a P-base region. The trench has a depth of not greater than 0.5 microns. The trench gate in such a structure is used to increase channel density and apparently does not effect the electric field distribution in the device. Iwamatsu does not have a structure that will result in uniform electric fields along the trench bottom walls. Thus, the MOSFET device of Iwamatsu is unable to obtain a high breakdown voltage.
Conventional lateral MOSFETs having such a trench gate structure suffer from a relatively high specific on-resistance at a given breakdown voltage due to non-uniform electric fields along the sidewalls and bottom of the trench. Thus, conventional trench gate lateral MOSFETs have specific on-resistance much higher than that of an ideal vertical power MOSFET.